Australian beaches could struggle to recover if a third La Nina weather event occurs this year after many of the most popular sandy stretches suffered back-to-back weather events that stripped away their sand.

As more extreme and intense weather events occur, once protected areas of beaches will become more exposed, threatening coastal communities, experts warn.

Dr Mitchell Harley, a senior lecturer at the University of NSW Water Research Laboratory, said Australian beaches are very dynamic and tend to fluctuate depending on the season. During storms, beaches lose more of their sand, while in warmer months beaches usually grow.

“We’ve been measuring Collaroy and Narrabeen beach for 46 years and, with those measurements, there have been fluctuations over a huge amount of space – up to 100 metres back and forth. The coastline is breathing in and out but when we look at long-term records they have generally been recently stable,” he said.

“During big storm events, like what we have seen over the past couple of years, what we see is that the waves strip the beaches of sand. It doesn’t disappear, it just moves into deeper water. The sand slowly returns to the beach during calming conditions.”

In normal summer months, waves move from a south or south-east direction. This typically means the northern end of the beach gets narrower and the southern ends get larger.

But during La Nina events, they shift slightly anti-clockwise and are more easterly, which leads to a higher risk of more erosion on the beach, particularly over the summer, and also “beach rotation” – where the beach realigns itself to the prevailing wind direction.

As a rule of thumb, it takes between five and ten days for every metre of sand from the shoreline to return to the beach, Harley said, and recovery can sometimes take months if beaches lose up to 40 metres of sand. But if there are back-to-back storm events, such as what has occurred over the past two years with La Nina, beaches may struggle to recover and begin to threaten infrastructure.

“If we do see more frequent events we are going to see continual pressure on beachfront properties,” he said. “Everyone wants to live next to the coast and there is always pressure to build more and more properties along the coast – but it creates legacy issues that generations in the future will have to address – particularly around climate change and the tremendous threats that can cause to the coastline.”

For example, wave sizes and sea levels will increase as oceans continue to warm, which will cause greater damage to beaches.

The coastline around Sydney is expected to experience between 20 centimetres and just over a metre of sea level rise in the next seven decades. This means that the state’s coastline can be expected to change significantly as climate change intensifies.

Meanwhile, tropical cyclones are likely to damage once protected areas of the coastline as they move further down the NSW coastline.

“We haven’t seen that type of erosion in the past and this could create new erosion hotspots and so that is a big concern,” Harley said.

Harley is part of a team of researchers involved in a citizen science project which monitors beaches to gain a deeper understanding of how coastlines change over time.

Mark “Dippy” DePena, 67, has surfed the shores of Cronulla Beach since high school and said the coastline has changed dramatically in the past 55 years.

“The sea doesn’t forgive. Honestly, it’s very powerful,” he said. “We’ve had some weird weather but this is the worst I’ve seen Cronulla since 1974 and we are really starting to get concerned,” he said. “They had to move the lifeguard tower – physically dismantle it – because it was starting to go under.”

DePena is unsure if his beloved Cronulla Beach can withstand much more without mitigation action, but he is determined to do all he can to preserve the beach – or what’s left of it. Twelve years ago, he teamed up with fellow surfer and friend, Andrew Pitt, to develop the Bate Bay Sand Placement Committee with the intention to improve the dedicated surf reserve and safeguard the shoreline.

“People want to live on the ocean, right? But these seas have actually stolen some of these front yards.”

Without proper mitigation and adaptation efforts from government, councils and community, ANU associate professor at Fenner School for Environment and Society Dr Liz Hanna said there would be impacts on people’s health. This includes displacement and loss of connection to the community.

“People will have to pay for their own coastal protection – some can afford it and some cannot,” she said. “Adaptation is tricky, holding back the sea is awfully hard.”

She suggested that, among other efforts, planned retreats need to be considered before people are forced to be displaced. “We don’t really know how much time we have but we can all agree that twiddling thumbs is not the right answer,” she said.

The NSW government is working with local councils to plan for and respond to erosion, including monitoring what new developments are allowed to go ahead, ensuring the environmental and community benefits remain and providing funding for mitigation works.

The Department of Environment and Heritage also provides seven long-term offshore wave buoys and several ocean level recorders to measure the changes.

Over the next 50 years, the Insurance Council of Australia has estimated governments will need to invest at least $30 billion in coastal protection and adaptation projects.

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“As these events increase in frequency and intensity, a growing number of exposed properties in Australia will become uninhabitable,” a council spokesperson said. “Insurance coverage is limited in these areas due to the high and growing risks, creating a protection gap.”

The council found in a report released last year that governments across all levels would need to invest at least $30 billion in large-scale coastal protection and adaptation projects over the next 50 years. The CSIRO’s decadal megatrends report, published last week, found that 150 million people worldwide live on land that could be vulnerable to future sea-level rise by 2050.

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